1. Field of the Invention
This invention relates to a pulse width-modulation control device for invertor devices, and more particularly to a pulse width-modulation control device for an invertor device of the pulse width-modulation type for use in portable AC power units.
2. Prior Art
In recent years, portable AC power units are increasing in number which are equipped with invertor devices for stabilizing the output frequency. For example, a portable power unit which generates AC power having an commercial frequency by means of a AC generator driven by an engine is known, e.g. by Japanese Provisional Utility Model Publication (Kokai) No. 59-132398, in which the engine is operated within a high speed range to cause the AC generator to generate high AC output, the output is converted into direct current, then the direct current is converted into alternating current by an invertor device, and the alternating current is outputted.
In the meanwhile, there is a demand for AC power units of this kind whose output waveform is as close to a sine wave dependent upon their application as possible. To this end, an attempt has been made to employ an invertor device of the pulse width-modulation type as the above-mentioned invertor device as disclosed in Japanese Provisional Patent Publication (Kokai) No. 60-82098.
According to this publication, a sine wave signal which determines the output frequency and the output voltage is compared with a triangular wave signal as a modulation carrier by a comparator, and a pulse width-modulated signal having a pulse width dependent upon the comparison result is supplied to the invertor device for switching control thereby. The sine wave signal has its amplitude corrected in a feedback manner responsive to detected output voltage.
However, as a result of the above feedback correction, the amplitude of the sine wave signal can be larger than that of the triangular wave signal, e.g. when the sine wave signal has an increased peak as a result of feedback correction to correct the waveform of the output voltage having a distorted peak. Consequently, the resulting pulse width-modulated signal is held at a high level or a low level. In such a case, if the invertor device is constructed such that the pulse width-modulated signal is supplied to a bridge circuit formed of FETS or the like via pulse transformers, the pulse width-modulated signal cannot be transmitted to the secondary side of the pulse transformers so that the inverter device cannot perform its switching operation.